Slight stumble off idle. Octane, jetting or sync?(updated)

[rider123]

Woah that is all over the map. Fast Larry's site says for the 15 pilot jet it's 2.5-3.0 turns out. In the Haynes manual It stipulates 3.0 turns out. Since mine was experiencing a slight lean stutter I backed it off another 1/8th. THis way if I need to go back to totally stock (3.0) it is very easy. already it's starting to run better just at idle and reving parked with only the outer carbs set to 3 and 1/8th. When I get home(and have tools) I will set the inner carbs to 3 and 1/8th out so that they are all the same and start there. I think I can get over the hump with a little experimentation. Who knows how weird the originals were set up as. I have a feeling the dealer set them to the absolute minimum for emmisions reasons (2.5) or maybe not at all. I can't believe I'm making it better by myself. I thought they were experts with real tools and exhaust sensors, etc. Well I guess the old saying "If you want something done right...." applies here. I let everyone know how it turns out. I'm going to make the bike slightly lean as it is colder outside so when it gets warm outside it won't be crazy rich. If I do figure it all out I'll post my experiences as I have a new bike so it may helps someone else avoid some frustration and hassle.


Edit: Well before I went home from work I thought I'd try the jets at 3.0 turns out which is stock on the two outer carbs. Wow what a difference. Obviously it's still a little weird because I couldn't get to the two inner carbs but it's alot better even though the temp is just over freezing and a little wet(scary drive!). So if the bike is running much better on two carbs at stock, hopefully it will be good on all 4 running stock. I think it will be a good starting point to tweak out any weirdness. I will probably leave it on stock untill next spring when I can tune it properly. However if running it 3 turns out is stock acording to Haynes and Fast Larry's site. What does the EPA consider "stock"? Also where was it before I got a chance to fix it? Where did the dealer put it? Strange....

[rider123]

Well 3.0 turns out seems to be the magic number, while slightly lean there is no stumble or anything like that. The idle now is slightly erratic but still ok. On the weekend I will try the best idle method on the pilots then I have to say it's fixed. It's only slightly lean now with it being really cold outside so I'm hoping when it warms up it in the spring it will be perfect. In the Haynes manual the US/UK use the same pilot jet screws and the manual lists the UK model (where its cold) as 3 turns out but lists the US model as "PRE_SET" it would be interesting to find out what it's preset to. I doubt it would be 3 turns out, one gentleman said his were set from .75 (!!) to 2.5 after he got the caps off. Talk about a difference!

Update: I've been thinking that the bike might even be a little rich it runs great cold to almost warmed up then gets a little sloppy when full hot. When I start the bike and let run a bit(no choke still warm enough to start) I can smell the faintest hint of fuel. As soon as its full hot you can't smell it anymore. Getting closer guys!!(finally, holy!)I'll turn her down an 1/8th at a time untill I figure she's really close then do the "best idle method".

[gyrogearcrunch]

I doubt it's the TPS on a new bike.  I have the same problem on a fairly new '03 with 4300 miles on it (Piped & jetted)

I''m pretty sure it's the carbs out of sync.  It's just an infrequent slight stumble that no one would notice except me.  Seems to happen most after I'm sitting at a stoplight for a long time.  (Like over a minute or so.)  The other 99.9% of the time it pulls like a mutha from idle to redline.  

Also, I kinda 'wrecked' my original carbs trying to get them out and jetting them.  I was lucky enough to get a complete new set (less than 100 miles on them) on Ebay in perfect condition, but who knows where the syncing is on them.

Carb syncing is one of the things on the 'to do' list after the first 600 miles or so.  I've yet to do it on mine.

I had a few furrin' cars with vacuum slide carbs on them, just like our Bandits use. I soon found out that it was a good idea to get one 'o them-thar vacuum sync-ing things that slapped over the carb's mouth in order to establish whether or not the whole bank of carbs was working together. Then it was a simple thing to adjust the butterfly on each carb to the same position as the other carbs. There was also a disabling pin used to raise the slide of one carb so that the jet-nut under the other carb could be used to adjust the mixture of that carb. This worked so well that I got almost 40 mpg on my 1962 TR-4, and the dyno said 80 RWHP! That was unheard of back in the day. Triumph TR-4's seldom got more than 70 RWHP.

Now I use a bank of vacuum gauges to do the same thing on my bikes, but there are no disabling pins that allow me to check the mixture, so that has be done by trial and error: Make a change in jetting, then road test. Keep it up long enough, and you will eventually find the sweet spot.

[Rocketjock]

Rider 123.
Reading back over your post, last one being Oct. I can only wonder why anyone would ride away with a new bike, find it stumbling and/ or misfiring and not immediately return to the dealer? Please don't misunderstand me, I am not trying to be critical here, just wondering why? As a fellow new (06) B12 owner, if I had detected anything but a perfect running bike, I would have gone back immediately. And after forking over a big lump of cash I'd make damn sure they would be footing the bill. It urked me when my 600 km checkup cost me $300. Secondly, my 06 B12 screw adjustment were factory set at 3.5 turns. Weren't yours? If not, then there may lie your problem. Still too lean.

[Bazza]

You said you don't want to mess with it right until the warranty is up.  

But I'll second what Ray said... go ahead and drill out those plugs and give the A/F screws a 1/2 turn out.  They are probably the most idle & off-idle related part of the carburetion... and they are set too lean from the factory.

Regardless... let us know the results of your TPS measurements.

Paul is correct. Your a/f screws really adjust fuel supply from idle to mid rpm range (where your needle jets kick in)

It is the 1st thing I would do. I bet a half turn out will cure your problem.

[CWO4GUNNER]

When I needed to know how carburetor adjustments work I used this online guide which is simple and basic.
http://www.motorcyclecarbs.com/carbs101.pdf
The pilot jet is the part which supplies most of
the fuel at low throttle openings. It has a small hole in
it which restricts fuel flow though it. The pilot jet affects carburetion from idle to around 1/4 throttle.